Foreign and domestic contributions to springtime ozone over China

China is facing a severe ozone problem, but the origin of its ozone remains unclear. Here we use a GEOS-Chem based global-regional two-way coupled model system to quantify the individual contributions of eight emission source regions worldwide to springtime ozone in 2008 over China. The model reproduces the observed ozone from 31 ground sites and various aircraft and ozonesonde measurements in China and nearby countries, with a mean bias of 10 %-15% both near the surface and in the troposphere. We then combine zero-out simulations, tagged ozone simulations, and a linear weighting approach to account for the effect of nonlinear chemistry on ozone source attribution. We find considerable contributions of total foreign anthropogenic emissions to surface ozone over China (2-11 ppb). For ozone of anthropogenic origin averaged over China, foreign regions together contribute 40 %-60% below the height of 2 km and 85% in the upper troposphere. For total ozone contributed by foreign anthropogenic emissions over China at various heights, the portion of transboundary ozone produced within foreign emission source regions is less than 50 %, with the rest produced by precursors transported out of those source regions. Japan and Korea contribute 0.6-2.1 ppb of surface ozone over the east coastal regions. Southeast Asia contributes 1-5 ppb over much of southern China and South Asia contributes up to 5-10 ppb of surface ozone over the border of southwestern China; and their contributions increase with height due to strong upwelling over the source regions. The European contribution reaches 2.1-3.0 ppb for surface ozone over the northern border of China and 1.5 ppb in the lower troposphere averaged over China. North America contributes 0.9-2.7 ppb of surface ozone over most of China (1.5-2.1 ppb over the North China Plain), with a China average at 1.5-2.5 ppb at different heights below 8 km, due to its large anthropogenic emissions and the transport-favorable midlatitude westerly wind. In addition to domestic emission control, global emission reduction is critical for China's ozone mitigation.